The present invention relates to a method of determining a scaling factor for scaling an output of an electrochemical cell. The scaling factor is for use in compensating for the effect on the output caused by any variation in a value of a property of a working electrode of the first cell from a reference value of that property.
In one embodiment the property is the thickness of the working electrode.
It will be understood that in the context of this patent “thickness” refers to the critical dimension of the working electrode. In one embodiment this is the width of a microband.
Sensors comprising electrochemical cells having micro-electrodes are used to detect or measure the concentration of a particular compound in a test sample. A biosensor comprising such a cell may for example be used to screen blood, urine or other such biological fluids for determination of their contents.
A cell having a micro-electrode and which may be used in a biosensor is described in our co-pending application WO 03/056319. The cell described in WO/03/056319, comprises a working electrode and a pseudo-reference electrode, at least one of which, typically the working electrode is a micro-electrode. A micro-electrode has at least one dimension not exceeding 50 micro meters. The working electrode is in the form of a band around the walls of the cell. An insulating material separates the working and counter electrodes. An electro-active substance is located in the cell and on contact of a measurement sample with the electro-active substance an electrochemical reaction may occur which may induce a measurable current, voltage or charge in the cell.
The electrochemical response of a microelectrode is a function of the electrode dimensions. For an electrochemical cell using a micro-band working electrode the thickness or width of the electrode constitutes the smallest dimension of the microelectrode dimensions. The measurable current also depends upon the thickness of the working electrode, so it is important to normalise for the variations in the thickness of the working electrode in order to accurately correlate the measured current to the concentration of the analyte.
In a biosensor the measurable current generated by an electrochemical reaction varies in a known relation to the concentration of the analyte under test. This known relation depends upon the thickness of the working electrode.
Currently, one approach for ensuring tight thickness tolerances for the working electrodes is to deposit the working electrodes using high vacuum techniques. Such techniques provide a uniform film deposition and are accurate. They are however economically expensive.
Other cheaper techniques for depositing working electrodes, for example, screen printing, ink jet printing, laminating, doctor blading and low vacuum deposition are less precise and provide working electrodes of unknown thickness and which vary in thickness across the sheet on which they are formed.
Embodiments of the present invention aims to alleviate the above-mentioned problems.